Preheat oven to 300 degrees F (150 degrees C).,Bring a large pot of lightly salted water to a boil; cook cauliflower until tender yet firm to the bite, about 10 minutes. Add green beans; cook until tender, about 5 minutes. Drain.,______,Cook and stir onion in the skillet with bacon grease over medium heat until tender and lightly browned, about 10 minutes.,Mix cauliflower, green beans, bacon, and onion together in a casserole dish.,Stir butter and mayonnaise together in a bowl; pour over cauliflower mixture. Stir to coat; top with mozzarella cheese.,Bake in the preheated oven until cheese is melted and bubbling, 20 minutes.